The present invention relates to a spark-ignition direct injection engine.
Techniques for improving both an exhaust emission performance and a thermal efficiency have been known. For example, JP2007-154859A discloses such a technique using a combustion mode of compression to ignite mixture gas inside the cylinder. With engines where such compression-ignition combustion is performed, the compression-ignition combustion becomes combustion with a significant increase in pressure as the engine load increases, causing an increase of combustion noises. Thus, as disclosed in JP2007-154859A, even for the engines which perform compression-ignition combustion, within an operating range on a high engine load side, instead of the compression-ignition combustion, spark-ignition combustion by the operation of the ignition plug has generally been performed.
JP2009-197740A discloses an engine which performs compression-ignition combustion within a low engine load operating range with low engine speed similar to the engine disclosed in JP2007-154859A, in which the open timing of an intake valve is advanced within a part of the compression-ignition combustion performing range where the engine load is relatively high and the engine speed is relatively high so that burned gas inside the cylinder is blown back to the intake port side once and then the burned gas is introduced into the cylinder again along with fresh air. In this manner, the temperature of the burned gas decreases because of the fresh air. Thus, the temperature inside the cylinder deceases and the significant pressure increase due to the compression-ignition combustion can be suppressed. The art disclosed in JP2009-197740A is advantageous in expanding the range where the compression-ignition combustion is performed to the higher engine load side.
With engines which perform compression-ignition combustion, it has been desired to expand the range where the compression-ignition combustion is performed, the compression-ignition combustion being advantageous regarding the exhaust emission performance and thermal efficiency. However, as described above, the compression-ignition combustion becomes combustion with a significant increase in pressure (dP/dt) as the engine load increases, and because of the restriction of NVH (Noise Vibration Harshness), it is difficult to expand the compression-ignition combustion performing range to the higher engine load side.